This invention relates to a method of fabricating a semiconductor device having a buried insulation layer separated by ditches and a method of fabricating the same, and more particularly to a semiconductor device having a so-called SOI (silicon-on-insulator) structure with the semiconductor device disposed on a substrate on which a buried insulator region is formed by ion implantation and a method of fabricating the same.
Integrated circuits are being developed very rapidly with respect to higher density, higher speed, and larger capacity. In particular, an integrated circuit based on a complementary MOS circuit (CMOS) possessing both high speed and low power consumption is expected to play a central role in the field of almost all integrated circuits in a few years.
What is particularly promising among such CMOS technologies is the SOI-CMOS technology using the SOI structure. This is because, by applying the SOI structure into the CMOS technology, the latchup phenomenon can be eliminated, higher density is realized, the parasitic capacitance can be reduced, and higher speed and lower power consumption can be realized.
Much has been reported about the forming method of SOI structure. Above all, a method of forming an oxide film layer or a nitride film layer by implanting ions of oxygen or nitrogen into a silicon substrate is regarded as being closest to practical use.
The conventional process of forming a buried nitride film layer by nitrogen ion implantation is disclosed, for example, by G. Zimmer et al. in IEEE Transactions on Electron Devices, vol. ED-30, No. 11, p. 1515 (1983). In this example, first a silicon substrate is thermally oxidized, and a silicon oxide film of about 500 .ANG. is formed thereon. This is to prevent spattering and surface contamination on the silicon substrate surface which may occur at the time of nitrogen ion implantation. In succession, nitrogen ions are implanted at an acceleration energy of 150 KeV, and dose of about 2.times.10.sup.18 /cm.sup.2. At this time, nitrogen ions spread with a Gaussion like distribution with a projection range Rp of about 3500 .ANG. and deviation of about 900 .ANG.. Then, after removing the silicon oxide film, an oxide film of about several thousand angstroms is deposited on the silicon substrate by the CVD method as a cap, and is heated for several hours in an inert gas atmosphere (for example, nitrogen, argon) at 1100.degree. C. to 1200.degree. C. By this heat treatment, the implanted nitrogen atoms react with the neighboring silicon atoms, and a buried nitride film region is formed. Excessive nitrogen atoms diffuse toward the surface or back of the substrate, and react with silicon to form part of the nitride film region. Thus, a nitride film region of about 2500 .ANG. is formed, and a silicon region insulated and separated from the silicon substrate is obtained.
When a CMOS circuit is formed in the above composed SOI structure, it is known to be free from latchup and low in parasitic capacitance, but, to the contrary, the leakage current at the junction is greater as compared with the bulk device, and the transconductance g.sub.m (g.sub.m =.DELTA.I.sub.DS /.DELTA.V.sub.GS) is small, and fluctuations of device characteristics are large. As one of their causes, shortness of carrier life time due to crystalline defects and distortions existing in the insulated and isolated silicon region is considered. When a buried insulation film is formed by implanted oxygen or nitrogen ions into the silicon substrate, a volumetric expansion of as much as half of the insulation film occurs, and the stresses in the silicon caused due to this volumetric expansion are not always isotropic, which is considered to cause such distortions and crystalline defects.